The major pathological hallmarks of Alzheimer's disease (AD), a progressive neurodegenerative condition leading to loss of memory, are characterized by the appearance of senile plaques which are primarily composed of Aβ and neurofibrillary tangle aggregates (Selkoe, 1997; Roberson and Harrell, 1997). Aβ, a 40-42 residue peptide, is derived from a larger protein, βAPP (695-770, amino acids), whose biological functions remain to be fully determined but whose pathological role may be separated on the basis of its final proteolysed form (Checker, 1995; Selkoe, 1997). βAPP derivatives are generated by three enzymatic activities termed α-, β- and γ-secretases, to produce different protein fragments that are either neuroprotective or amyloidogenic. An aspartyl protease with β-secretase like properties has been identified (Hussaain et al., 1999; Sinha et al., 1999; Vassar et al., 1999; Yan et al., 1999), that may serve as a therapeutic marker. However, its value as a target for drug development is complicated by its location within two membranes (plasma and Golgi apparatus). Furthermore, the role of alternative compensatory activities remains unclear. Indeed, a second enzyme, Thimet oligopeptidase, was found capable of β-secretase activity in transfected COS cells (Koike et al., 1999). A major pharmaceutical industry focus has been to look for agents that reduce amyloidogenic processing using compounds that can manipulate βAPP to produce non-amyloidogenic by-products. However, it is important to note that the role of alternative βAPP fragments in AD is unclear.
Regarding regulatory mechanisms involved in βAPP processing, environmental agents have been demonstrated to accelerate βAPP turnover into its pathological Aβ form (Selkoe, 1997). Furthermore, the cellular surrounding of neurons, particularly astrocytes and microglia, are additional and non-neuronal sources of βAPP (Funato et al., 1998; Akiyama et al., 2000). Thus, amyloid plaque occurrence is often associated with enlarged microglia which produce interleukin-1 (EL-1), a potent mediator of astroglial proliferation and βAPP production (Akiyama et al., 2000). The fact that IL-1 can influence this process suggests that signaling pathways induced by cytokines are interconnected with βAPP metabolism. Another example of receptor-signaling association and βAPP homeostasis is demonstrated through the activation of muscarinic m1 and m3 receptors which modify βAPP synthesis and processing through MAP kinase dependent and independent pathways (Felder et al., 1993; Nitsch et al., 1992 and 1994). Reductions in muscarinic receptors, as in Aβ, may alter βAPP metabolism and result in subsequent Aβ deposition. Cholinergic system impairment has been reversed with moderate success by the use of anticholinesterases (Greig et al., 1995; Brossi et al., 1996), the only approved drugs for Aβ treatment.
A family of novel anticholinesterases, phenserine and analogues, has been synthesized. Phenserine dramatically improves cognitive performance in rodents and is in clinical trails (Greig et al., 1995; Patel et al., 1998). Studies of rats with forebrain cholinergic lesions that are known to dramatically increase βAPP in cholinergic projection areas have shown that phenserine can protect against this and additionally, reduce βAPP production in naive animals (Haroutunian et al., 1997). As both βAPP processing and cholinesterase activity are affected in the Aβ brain (Bronfman et al., 1996) and as the anticholinesterase, tacrine, has been shown to decrease βAPP and Aβ in neuronal cells in vitro (Lahiri et al., 1998), current studies have focused on the molecular changes induced by phenserine. In these studies, naturally-occurring phenserine (the (−)-enantiomer) was used.
It is the cholinergic action of anticholinesterases such as (−)-phenserine, rivastigmine (Exellon®, Novartis®), donepezil (Aricept®, Pfizer®), galanthamine (Jansen®), tacrine (Cognex®, Warner Lambert®), (−)-physostigmine (Synapton®, Forest®), that provides the compounds their ability to improve cognitive performance in both animal models and humans. Likewise, it is the cholinergic action that is also dose limiting for these same compounds (nausea, sweating, GI effects) (Becker et al., 1991). Conversely, the (+)-enantiomers are unable to inhibit either acetylcholinesterase (AChE., EC 3.1.1.7.) or butyrylcholinesterase (BChE., EC 3.1.1.8.), and hence have no cholinergic action. The (+)-enantiomers are also unnatural isomers and thus, need to be synthesized. Synthetic procedures provide a mixture of (+)- and (−)-forms that require early separation into optically pure forms to eventually obtain the final products.
Additional advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.